T and B Lymphocytes: Cellular and Humoral Responses Revision Notes | A Level AQA Biology

1. The Immune System's Command Structure

Your body is invaded by a pathogen. Within hours, a highly coordinated response begins — but it does not start with antibodies or killer cells. It starts with a cell that does something deceptively simple: it presents a fragment of the invader to the rest of the immune system.

This lesson covers two parallel arms of the adaptive immune response: the cellular response, driven by T lymphocytes, and the humoral response, driven by B lymphocytes. Both arms are triggered by the same starting event — antigen presentation — and both are essential for clearing an infection.

1a. Antigen-Presenting Cells: The Alarm Raisers

When a phagocyte such as a macrophage engulfs a pathogen, it does not destroy it and move on. It digests the pathogen and displays fragments of its antigens on its own cell surface membrane, bound to proteins called MHC molecules. This converts the macrophage into an antigen-presenting cell (APC).

Picture the APC as a soldier holding up a wanted poster. The poster (the antigen fragment) is shown to passing T lymphocytes. When a T lymphocyte with a complementary receptor encounters that antigen, it becomes activated. Without this presentation step, the adaptive immune response cannot begin.

The APC activates a specific type of T lymphocyte called the helper T cell (TH cell). The helper T cell is the central coordinator of the entire adaptive response — it does not kill pathogens directly, but it sends chemical signals that activate every other branch of the immune system.

Antigen presentation is the gateway to the entire adaptive immune response — without it, neither T cells nor B cells can be activated. This spec point underpins everything that follows in this lesson.

2. The Cellular Response: T Lymphocytes in Action

Once a helper T cell is activated by an APC, it divides rapidly by mitosis to produce a clone of identical helper T cells. These cells then release signalling proteins called cytokines, which act on three key targets: cytotoxic T cells, B cells, and phagocytes.

• Helper T cells stimulate cytotoxic T cells (T_C cells) to divide and become active killers of infected or foreign cells.
• Helper T cells stimulate B cells to divide and differentiate, driving the humoral response.
• Helper T cells stimulate phagocytes, enhancing their ability to engulf and destroy pathogens.

The helper T cell is therefore the linchpin of the adaptive immune response. Losing helper T cells — as occurs in HIV infection — cripples both the cellular and humoral arms simultaneously, which explains why HIV leads to immunodeficiency.

2a. Cytotoxic T Cells: The Targeted Killers

Cytotoxic T cells (TC cells) are activated when helper T cells release cytokines in their presence. Each cytotoxic T cell carries a receptor that is specific to one particular antigen — it will only attack cells displaying that antigen on their surface.

When a cytotoxic T cell binds to an infected body cell displaying the target antigen, it releases proteins called perforins, which punch holes in the target cell membrane. This triggers apoptosis — programmed cell death — destroying the infected cell before the pathogen inside can replicate further.

This is the essence of the cellular response: T lymphocytes respond to foreign antigens by producing clones of cytotoxic T cells that directly destroy cells harbouring the antigen. The response is cell-mediated, meaning it targets infected cells rather than free-floating pathogens.

The cellular response is particularly important for dealing with intracellular pathogens — viruses and some bacteria that hide inside host cells, where antibodies cannot reach them. Cytotoxic T cells solve this problem by targeting the infected cell itself.

3. The Humoral Response: B Lymphocytes and Antibodies

While cytotoxic T cells handle infected cells, a parallel response targets free-floating pathogens and their toxins. This is the humoral response, carried out by B lymphocytes and the antibodies they produce.

Each B lymphocyte carries surface receptors that are specific to one antigen. When a B cell encounters its complementary antigen — and receives the activating cytokine signal from a helper T cell — it undergoes clonal selection: the specific B cell is selected and stimulated to divide by mitosis, producing a large clone of identical cells.

The cloned B cells then differentiate into two populations. The majority become plasma cells, which are highly specialised antibody-secreting factories. A smaller number become memory B cells, which persist in the body long-term and enable a faster secondary response upon re-exposure to the same antigen.

3a. Antibody Structure

Plasma cells release antibodies into the blood and lymph. An antibody is a glycoprotein with a specific structure that allows it to bind to one particular antigen with high specificity.

Picture a Y-shaped molecule. The antibody consists of four polypeptide chains: two identical long chains (heavy chains) and two identical short chains (light chains), held together by disulfide bonds. The tips of the Y — the variable regions — form the antigen-binding sites. Because each antibody has two binding sites, it is described as bivalent.

The variable region is the key to specificity. Because its amino acid sequence differs between antibodies, each antibody has a uniquely shaped binding site that is complementary to only one antigen. This is the molecular basis of the immune system's ability to distinguish self from non-self.

4. Antigen-Antibody Complexes and Pathogen Destruction

When an antibody's variable region binds to its complementary antigen, an antigen-antibody complex forms. This binding is highly specific — the shape of the variable region is complementary to the antigen, much like an enzyme and its substrate. The complex does not destroy the pathogen directly; instead, it marks it for destruction by other mechanisms.

AQA requires you to know two specific mechanisms by which antigen-antibody complexes lead to the destruction of bacterial cells: agglutination and phagocytosis.

• Agglutination: Because each antibody has two antigen-binding sites, it can bind to two bacterial cells simultaneously. Multiple antibodies cross-link many bacteria together, forming large clumps (agglutinates). Clumped bacteria cannot spread through tissues and are far easier for phagocytes to engulf.
• Phagocytosis: Opsonisation is the process by which antibodies coat a pathogen. Phagocytes have receptors (Fc receptors) that recognise and bind to the constant regions of these antibodies, making it easier for the phagocyte to engulf and destroy the pathogen by phagocytosis. Thus, opsonisation greatly enhances the rate of phagocytosis by marking pathogens for ingestion.

Worked example: Tracing the Destruction of a Bacterial Cell: From Antigen to Elimination

Describe, in sequence, how a bacterial cell entering the body is ultimately destroyed via the humoral response, from antigen presentation through to phagocytosis.

1. A macrophage engulfs the bacterium, digests it, and displays bacterial antigen fragments on its surface via MHC proteins, becoming an APC. — This is the essential first step — without antigen presentation, neither T cells nor B cells can be activated.
2. A helper T cell with a complementary receptor binds to the antigen on the APC. The helper T cell is activated and divides by mitosis, releasing cytokines. — Cytokines are the chemical signals that coordinate the rest of the response.
3. A B cell with a surface receptor complementary to the bacterial antigen binds to the antigen and receives cytokine signals from the helper T cell. It undergoes clonal selection — dividing by mitosis to produce a clone of identical B cells. — Only the B cell with the correct receptor is selected; this ensures specificity of the response.
4. The cloned B cells differentiate into plasma cells, which secrete large quantities of antibodies specific to the bacterial antigen. — Each plasma cell can secrete thousands of antibody molecules per second.
5. Antibodies bind to antigens on the bacterial cell surfaces, forming antigen-antibody complexes. Because each antibody is bivalent, it cross-links bacteria together, causing agglutination. — Agglutinated bacteria cannot spread and present a larger, easier target for phagocytes.
6. Phagocytes bind to the constant regions of antibodies coating the bacteria (opsonisation) and engulf the agglutinated clumps by phagocytosis, destroying the bacteria. — The antibody coating dramatically increases the efficiency of phagocytosis — this is why the humoral response is so effective against extracellular bacteria.

Answer: The bacterium is destroyed through a coordinated sequence: APC activation of helper T cells → cytokine release → clonal selection of B cells → plasma cell differentiation → antibody secretion → antigen-antibody complex formation → agglutination → enhanced phagocytosis.

5. Comparing the Cellular and Humoral Responses

Both responses are triggered by the same antigen and both require helper T cell activation, but they differ in their targets and mechanisms. Understanding this comparison is a common exam demand — 'compare' questions require you to state both similarities and differences explicitly.

A key similarity worth stating in exam answers: both responses involve clonal expansion — the selected lymphocyte divides by mitosis to produce a large clone of identical cells, ensuring a rapid and amplified response.

6. Exam Technique and Mark-Winning Strategies

This topic generates a predictable set of exam questions. The marks are often lost not through ignorance of the biology, but through imprecise language. The guidance below targets the most common errors seen in mark schemes.

Both the cellular and humoral responses depend on the same foundational sequence — antigen presentation, helper T cell activation, and clonal expansion — but diverge in their effector mechanisms. Mastering this distinction, and expressing it with precise AQA terminology, is what separates a Band 1 answer from a Band 3 answer in extended-response questions.